Conventional electrophotographic apparatus, such as copiers, printers, facsimile machines, etc., comprise an imaging surface, such as a photoconductive element, normally in the form of a drum or belt. Arranged in timed sequence around the imaging surface are a plurality of processing stations for performing various functions. These processing stations may comprise stations for charging the imaging surface, electrostatically forming a latent image on the imaging surface, developing the latent electrostatic image with a developer commonly referred to as toner, transferring the developed image from the imaging surface to a substrate such as paper, feeding paper to the transferring station, cleaning the imaging surface, i.e., removing residual toner on the imaging surface, and fixing the transferred developed image on the paper.
A typical reproduction operation comprises charging the imaging surface, such as a photoconductive drum, and exposing the charged surface to a light pattern of an original image to be reproduced thereby selectively discharging the imaging surface in accordance with the original image. The resulting pattern of charged and discharged areas on the surface of the photoconductive drum forms an electrostatic charge pattern or electrostatic latent image conforming to the original image.
The latent electrostatic image is developed by contacting it with finely divided toner which is held by electrostatic force on the imaging surface. The toner image is transferred to a substrate, such as paper, in a transferring device into which paper is fed by a registration roller toward the drum in synchronization with drum rotation. As the leading edge of the paper abuts the drum, electrostatic forces adhere the two together, and the transferring device transfers a toner image from the photoconductive drum to the paper. After transfer, the toner image is fixed to form a permanent record.
Subsequent to development, and after transfer of the developed image to the paper, some toner inevitably remains on the photoconductive drum, held thereto by electrostatic and/or Van der Wals force. Additionally, other contaminants, such as paper fibers, toner additives, Kaolins and various other forms of debris, have a tendency to be attracted to the charge retentive surface.
Contemporary commercial automatic copiers/reproduction machines comprise an electrostatographic imaging surface, which may be in the form of a drum or belt. The imaging surface moves at high rates in timed unison relative to a plurality of processing stations. This rapid movement of the electrostatographic imaging surface requires vast amounts of toner to be employed during development. Associated with the increased amounts of toner is the difficulty in removing residual toner remaining on the imaging surface subsequent to transfer.
One type of device conventionally employed for charging the imaging surface of a photoconductive member is a corona charger normally positioned slightly spaced apart from the surface of the imaging surface for applying a surface charge thereto. Typically, a corona charging device comprises a wire electrode and a shield electrode to which is normally applied a relatively high voltage, on the order of 4 to 8 kilovolts, to induce 500 to 800 volts of surface potential on the imaging surface. Corona chargers are of relatively low charging efficiency, because most of the discharging current from the wire electrode flows to the shield electrode, leaving a small percentage of the total discharging current flowing to the imaging member to be charged. Another disadvantage attendant upon employing a corona charger is the generation of ozone which constitutes a health hazard and is, therefore, environmentally undesirable. Accordingly, when employing a corona charger it is necessary to install filtering and air distribution systems in any environment in which the electrostatographic apparatus is situated. In addition, image blurring occurs as a result of the oxidation of the image transfer components and deterioration of the photoconductive surface. Still another disadvantage attendant upon employing a corona charger is contamination of the wire electrode by fine dust attracted by the electrostatic field created by the electrode, thereby necessitating periodic cleaning and/or replacement of the wire electrode.
The disadvantages associated with corona chargers have led to the implementation of alternatives to the corona chargers, such as a contact type charge inducing member as disclosed in Japanese laid open 3-130,787. The disclosed system comprises a contact charge inducing member which is maintained in contact with the surface of a charge receiving member, e.g., a photoconductive drum, thereby charging the photoconductive drum at an advantageously relatively low voltage. Since a discharge is not established, ozone is not generated and the accumulation of dust on the wire electrode avoided.
As shown in FIG. 1, the prior art apparatus comprises photoconductive drum 60, a contact charge inducing member in the form of charging roller 62 connected to a relatively low voltage power supply 64 via conductive spring 61. The apparatus also comprises cleaning element 63 which is urged into contact with the surface of charging roller 62 upon energizing solenoid 65. Cleaning element 63 is made of felt, or a suitable foam such as a polyurethane, or a suitable elastomer such as an ethylene-propylene-diene-monomer (EPDM) elastomer. Solenoid 65 enables periodic movement of cleaning element 63 into and out of contact with charging roller 62.
In operation, solenoid 65 is normally off so that the armature extends out of solenoid 65 and cleaning element 63 is spaced apart from, i.e., out of contact with, charging roller 62. During operation, toner and other contaminants inevitably accumulate on charging roller 62, as from the surface of drum 60, decreasing its charge inducing efficiency. In addition, such toner and other contaminants tend to redeposit on photoconductive drum 60, resulting in poor quality reproductions. When solenoid 65 is switched on, the armature is drawn into the solenoid, extending cleaning element 63 into contact with charging roller 62 to remove toner and other contaminants therefrom while charging roller 62 rotates due to frictional engagement with photoconductive drum 60.
Another prior art cleaning element is disclosed in Japanese laid open 3-101,768. The cleaning element is also made of felt or other suitable materials, such as polyurethane foam or rubbers.
A conventional charging roller 62, as shown in FIG. 2A, normally comprises a conductive metal core 65 surrounded by a layer of elastomeric material 62a, such as rubber or an elastomeric resin, and a surface layer 62b having a thickness in the range of about 4 to about 14 microns and a hardness greater than that of underlying layer 62a.
Because the underlying layer 62a of elastomeric material is inherently formed with surface irregularities, as shown in FIG. 4, the outer surface layer 62b conforming to the shape of the underlying layer, is also irregular. This inherent irregular outer surface layer 62b is characterized by a convex and concave surface topography comprising crevices, recesses, etc., renders it particularly receptive to the accumulation of embedded or lodged finely divided material such as toner and other contaminants. Toner is a particularly troublesome contaminant, since its particle size is such that it easily penetrates crevices on the surface of a charge inducing member so that the toner tends to accumulate in the concave portions.
With reference to FIG. 2B, despite the use of the prior art cleaning elements, which were basically stationary while the charging roller rotates, toner and other contaminants (Tn) inevitably accumulate and lodge in crevices and recesses on the irregular surface of charging roller 62 (FIG. 2C). Such Tn tend to become embedded or lodged between charging roller 62 and cleaning element 63 as shown in FIG. 3, resulting in the accumulation of Tn on the surface of charging roller 62. In addition, the accumulation of Tn between cleaning element 63 and charging roller 62 creates friction on the surface of charging roller 62 thereby disadvantageously imparting vibrations to the photoconductive element resulting in poor quality reproduction. After a period of time, the accumulated Tn causes nonuniform charging resulting noticeably poor quality reproductions.
With reference now to FIGS. 5A and 5B, an area 70 of the surface of charging roller 62, having the irregular surface as shown in FIG. 4, has been cleaned by a prior art rotational element 63. The surface is characterized by an overlapping area 71 which has not been effectively cleaned by the prior art rotational cleaning element 63 due to poor contact therebetween because of the irregular surfaces of both the cleaning element and rotational element. This is because area 71 is in the "shadow" of area 70 and will not be "seen" by element 62 as it sweeps over the rotational element 63. Thus, as illustrated in FIG. 6A, accumulated Tn will remain embedded in surface crevices and recesses 62C, even after cleaning.
Several prior art techniques have been developed to remove toner and other contaminants from a photoconductive drum after transfer of the developed image to a substrate. See, for example, Japanese laid open 60-134275.